Gynecologists and pediatricians largely supported a U.S. Food and Drug Administration advisory panel's vote on Wednesday to approve the vaccine Gardasil for use in males aged 9-26 to protect against genital warts. But some say a conservative approach should be taken in vaccinating boys until more evidence is in.

The votes are non-binding, but the FDA usually follows an expert panel's advice.

Merck, the company that manufactures the vaccine, which protects against four common strains of the human papilloma virus (HPV) and has been approved to prevent genital lesions and cervical cancer in women, has consistently presented strong evidence supporting the vaccine's efficacy in males.

But lacking key information about the vaccine's duration and potential to protect against cancer in males, experts are adopting a wait-and-see attitude towards the drug until the FDA and some professional societies make formal recommendations for Gardasil use.

"[The data] looked very convincing so I wasn't really surprised that the recommendation was favorable," said Dr. Kevin Ault, associate professor of Gynecology and Obstetrics at Emory University. "It's true that [genital warts] are not cancer but they're difficult to treat and socially stigmatizing. That might be a trigger for more widespread use."

Some Doctors Urge Cautious Approach on Gardasil in Boys

ABC News' Chief Medical Editor Dr. Timothy Johnson said he favored a conservative approach in light of the panel's recommendation.

"It's going to be a really tough choice for parents and they should make that decision on a family basis," Johnson said. "I'm not prepared to say all boys in that age group should get it without question."

One of the goals for vaccinating males was the potential to reduce HPV transmission to females. In the U.S., 6.2 million cases of HPV are diagnosed each year in men and women, according to the Centers for Disease Control and Prevention.

"The HPV virus doesn't spring from the air to the genitals of males and females, boys and girls. They are spreading it to each other," said Dr. Joseph Zanga, chief of Pediatrics at Columbus Regional Healthcare System. "We should try our best to protect them from inconvenient and potentially very dangerous diseases... But if it doesn't do that, then it will be useful in preventing an infection for boys but won't help girls."

But preventing men from HPV infections that could lead to head and neck, anal, and penile cancers -- those being the most common HPV-related cancers in men -- could be appealing, particularly since there is no infrastructure for regular screening of such cancers the way that regular pap smears have helped prevent cervical cancer in women.

"It's under-recognized how important HPV is as a disease causing infection in men," said Dr. Richard M. Haupt, head of the clinical program for Gardasil at Merck.

But no data shows that Gardasil offers direct protection against cancer. Instead, the vaccine has the potential to be protective based on its ability to prevent the persistent HPV infections that can be precursors to cancer.

"We can't demonstrate it directly yet but we believe it has the same cancer-protective effect in men and boys [as girls]," said Dr. Joel Palefsky, associate dean for Clinical and Translational Research at the University of California at San Francisco and co-lead investigator for Merck's study on Gardasil in males. "It's a public health intervention to offer to boys as well as girls."

More Research Needed?

Beyond its ability to reduce HPV transmission between males and females or prevent cancer, further unanswered questions about Gardasil may give experts pause.

For example, Gardasil has been proven effective in males for up to three years and in females up to five years, but not beyond.

"Just because we have data up to a point doesn't mean that the vaccine will stop working after that," said Anna Giuliano, chair of the department of Cancer Epidemiology and Genetics at the H. Lee Moffitt Cancer Center in Tampa, Fla. and co-leader of the Merck study on Gardasil. "That is true of every new vaccine."

But without such data, clinicians may question the optimal age at which to administer Gardasil, and if booster shots will be necessary.

And experts said the potential reaction from parents will be mixed.

"Boys may be interested as they go into high school and college," said Dr. Diane Harper, director of the Gynecologic Cancer Prevention Research Group at University of Missouri. "But I don't think parents are going to be at all interested."

On the other hand, Dr. Lauren Streicher, an obstetrician-gynecologist at Northwestern Medical School, who supports use of the vaccine, said she vaccinates boys off-label because some parents ask for Gardasil for their sons as well as their daughters.

"Parents on board with getting their daughters vaccinated will be equally on board with getting their sons vaccinated," said Streicher, adding that the opposite would be true as well, that parents with no interest in having their daughters receive Gardasil would not be likely to vaccinate their sons.

Recommendations from the FDA and organizations such as the CDC's Advisory Committee on Immunization Practices, which will discuss Gardasil at the next meeting in October, as well as the American Academy of Pediatrics and the American Academy of Family Physicians will be crucial as clinicians, public health officials and families decide whether to use Gardasil to vaccinate boys against HPV.

Doctors Seeking More Answers on New Use for HPV Vaccine

Zanga noted that further research from Merck will help move the vaccine in the appropriate direction.

"I'd like to see the manufacturer present at the outset what the limits of protection are -- whether this is a boostable vaccine, whether it will require more doses, or whether having gotten it once, will it never be protective again," Zanga said. "Before we push this [vaccine] and make it a routine part of preadolescent and adolescent immunizations for boys, we ought to know the answers to these questions."